Dampening systems of the type disclosedd in U.S. Pat. No. 3,168,037 and U.S. Pat. No. 3,343,484 to Harold P. Dahlgren have offered significant improvements over dampening systems previously employed.
Such systems have included two rollers disposed in pressure indented relation, one of the rollers having a relatively hard hydrophillic surface and the other roller having a smooth resilient surface. In the preferred embodiment illustrated in the drawings of the aforementioned patents the transfer and metering rollers were geared together to travel at substantially equal surface speeds and were driven by a variable speed drive means for metering a film of dampening fluid through a nip between the rollers and for transferring the film of dampening fluid to the lithographic printing system.
The thickness of the film of dampening fluid delivered to the surface of the transfer roller moving out of the nip between the metering roller and transfer roller was controlled primarily by adjustment of pressure between adjacent surfaces of the metering and transfer rollers.
The rate at which the metered film of dampening fluid, carried on the surface of the transfer roller, was delivered to the lithographic printing system was controlled by the variable speed drive means. Briefly stated, the theory of operation was that given a film of predetermined thickness the quantity of dampening fluid delivered was directly related to the speed of the film. In other words, to reduce the rate at which dampening fluid was delivered to the lithographic system, the transfer roller speed could be reduced; and, to increase the quantity of dampening fluid, the speed of the transfer roller could be increased. However, such results follow only so long as the speed differential between the transfer roller and the ink coated form roller was not excessive.
Excessive slippage resulted in application of hydraulic forces in the nip between the transfer roller and the applicator roller which caused excessive emulsification of the distinct films of ink and dampening fluid. Excessive emulsification of ink and dampening fluid at the nip between the transfer roller and form roller resulted in transfer of the emulsion by the surface of the transfer roller to the surface of the resilient metering roller which was not hydrophillic. Build-up of ink on the surface of the metering roller resulted in streaking of printed sheets because of non-uniform surface characteristics of the metering roller which caused a non-uniform film of dampening fluid to be metered onto the surface of the transfer roller.
To accommodate existing press design, metering and transfer rollers have been constructed of diameters generally in a range of approximately 3 to 6 inches. At surface speeds of about 300 feet per minute films of dampening fluid tended to separate from the surface of the metering roller as a result of centrifugal force. When the metering roller, geared to the transfer roller, was slowed to prevent splashing and slinging of dampening fluid, excessive slippage resulted at the nip between the transfer roller and the form roller which carried the dampening fluid to the lithographic printing system.
In applications where the metering roll was driven by the hydrophillic transfer roller and where a relatively fast hydrophillic transfer roller surface speed was required for printing, such as in a web press, the metering roller slung water to such an extend that experiments were conducted on apparatus wherein the transfer roller and metering roller were geared together to run at a speed ratio of 2:1 thereby substantially reducing the surface speed of the metering roller which carried a thick film of dampening fluid. Slinging of dampening fluid was then stopped but the film of dampening fluid delivered by the transfer roller was of a thickness which required excessive slippage, resulting in excessive emulsification, between the hydrophillic transfer roller and the form roller.
Tests were conducted on a printing press having an ink coated form roller running at a surface speed of 1,000 feet per minute. The hydrophillic transfer roller and the resilient metering roller of the liquid applicator system were geared together at a speed ratio of 5:3. The liquid applicator system could not be adjusted to provide acceptable results because as the surface speed of the transfer roller was increased to prevent excessive slippage between adjacent surfaces of the transfer roller and the applicator roller too much water was delivered to the lithographic system. Increasing pressure at the nip between the metering roller and the transfer roller did not effectively reduce the thickness of the film of dampening fluid, carried by the transfer roller, to the required thickness.
A further test was conducted on the same printing press when the surface speed of the form roller was 1,000 feet per minute. However, the hydrophillic transfer roller and the resilient metering roller were driven by separate variable speed drive motors such that the transfer roller was run at a surface speed of 500 feet per minute and the metering roller was run at speeds of less than 50 feet per minute. Extremely high quality printing was produced.
In laboratory experiments the film thickness carried by the surface of the hydrophillic transfer roller was measured. The transfer and metering rollers were geared together at a 1:1 speed ratio and pressure between the rollers was maintained at a constant level. As surface speeds of the transfer and metering rollers were continuously increased the thickness of the film carried on the surface of the transfer roller did not continuously increase. A graph of the film thickness relative to surface speed of the metering and transfer rollers produced a curve of somewhat sinusoidal nature.
The same test was conducted with the transfer and metering rollers being geared together at a speed ratio of 2:1. Again the film thickness was somewhat sinusoidal in nature as surface speeds of the rollers were increased. Thus, transfer and metering rollers geared together at a fixed speed ratio do not deliver a uniformly increasing quantity of dampening fluid to a lithographic printing system as surface speeds of the rollers are increased over a wide range of speeds. As the surface speeds of the rollers is increased the quantity of dampening fluid increases to a point after which further increase in the surface speed of the rollers results in reduction in the quantity of dampening fluid delivered.
From the foregoing it is concluded that provision of separate variable speed drive means for independently controlling surface speeds of metering and transfer rollers of the systems of the type disclosed in the aforementioned Dahlgren patents permits metering of thinner films in precisely controlled quantities onto the surface of the transfer roller and permits adjustment of the surface speed of the transfer roller relative to the surface speed of an applicator roller to produce desired hydraulic forces in the nip between the transfer roller and the applicator roller to prevent excessive emulsification of dampening fluid and ink while delivering proper amounts of dampening fluid to the lithographic printing system.